feat: Add Gleam (1.5.x) gitignore

[vidbina]

this is useful for developers that init a gleam project into an existing git repo where the gleam new tool will refrain from populating the .gitignore file. This situation may occur for users of devenv.nix or flake.nix.

Reasons for making this change:
There is no clear way to force the gleam new CLI tool to populate a valid .gitignore when running the CLI tool from inside a git repo. Since I manage my build tooling with devenv.nix which already spawns a git repo, I needed a different way to figure out how to get a sufficiently decent default .gitignore for my Gleam project. As I didn't find anything or any relevant open PR (searched for any PR matching the filter: gleam) to inform this, I reckoned that having this recorded in the .gitignore repo provides folks a reference in case they run into a similar problem.

Links to documentation supporting these rule changes:

• https://gleam.run = language homepage
• https://gleam.run/writing-gleam/ = docs outlining file structure to be expected upon running gleam new which includes the .gitignore
• https://github.com/gleam-lang/gleam/blob/fcfc553/compiler-cli/src/new.rs#L116-L120 for the source that defines what goes into the .gitignore that the gleam new tool populates

[vidbina]

Hiya @dooleydevin lmk what I can do here to make the path to merging this easier for you. Afaict, this gitignore should cover things as I haven't encountered any need to amend it yet.